The present invention is related in general to the field of semiconductor devices and processes and more specifically to the fabrication of a molded package suitable generally for micromechanical devices and specifically for digital mirror devices.
Micromechanical devices include actuators, motors, sensors, spatial light modulators (SLM), digital micromirror devices or deformable mirror devices (DMD), and others. The technical potential of these devices is especially evident when the devices are integrated with semiconductor circuitry using the miniaturization capability of semiconductor technology.
SLMs are transducers that modulate incident light in a special pattern pursuant to an electrical or other input. The incident light may be modulated in phase, intensity, polarization or direction. SLMs of the deformable mirror class include micromechanical arrays of electronically addressable mirror elements or pixels, which are selectively movable or deformable. Each mirror element is movable in response to an electrical input to an integrated addressing circuit formed monolithically with the addressable mirror elements in a common substrate. Incident light is modulated in direction and/or phase by reflection from each element.
As set forth in greater detail in commonly assigned U.S. Pat. No. 5,061,049, issued on Oct. 29, 1991 (Hornbeck, xe2x80x9cSpatial Light Modulator and Methodxe2x80x9d), deformable mirror SLMs are often referred to as DMDs in three general categories: elastomeric, membrane, and beam. The latter category includes torsion beam DMDs, cantilever beam DMDs, and flexure beam DMDs. Each movable mirror element of all three types of beam DMD includes a relatively thick metal reflector supported in a normal, undeflected position by an integral, relatively thin metal beam. In the normal position, the reflector is spaced from a substrate-supported, underlying control electrode, which may have a voltage selectively impressed thereon by the addressing circuit.
When the control electrode carries an appropriate voltage, the reflector is electrostatically attracted thereto and moves or is deflected out of the normal position toward the control electrode and the substrate. Such movement or deflection of the reflector causes deformation of its supporting beam storing therein potential energy which tends to return the reflector to its normal position when the control electrode is de-energized. The deformation of a cantilever beam comprises bending about an axis normal to the beam""s axis. The deformation of a torsion beam comprises deformation by twisting about an axis parallel to the beam""s axis. The deformation of a flexure beam, which is a relatively long cantilever beam connected to the reflector by a relatively short torsion beam, comprises both types of deformation, permitting the reflector to move in piston-like fashion.
A typical DMD includes an array of numerous pixels, the reflectors of each of which are selectively positioned to reflect or not to reflect light to a desired site. In order to avoid an accidental engagement of a reflector and its control electrode, a landing electrode may be added for each reflector. It has been found, though, that a deflected reflector will sometimes stick or adhere to its landing electrode. It has been postulated that such sticking is caused by intermolecular attraction between the reflector and the landing electrode or by high surface energy substances adsorbed on the surface of the landing electrode and/or on the portion of the reflector which contacts the landing electrode. Substances which may impart such high surface energy to the reflector-landing electrode interface include water vapor or other ambient gases (e.g., carbon monoxide, carbon dioxide, oxygen, nitrogen) and gases and organic components resulting from or left behind following production of the DMD. A suitable DMD package is disclosed in commonly assigned U.S. Pat. No. 5,293,511 issued on Mar. 8, 1994 (Poradish et al., xe2x80x9cPackage for a Semiconductor Devicexe2x80x9d).
Sticking of the reflector to the landing electrode has been overcome by applying selected numbers, durations, shapes and magnitudes of voltage pulses to the control electrode. Detail can be found in U.S. Pat. No. 5,096,279, issued on Mar. 17, 1992 (Hornbeck et al., xe2x80x9cSpatial Light Modulator and Methodxe2x80x9d). Further improvement of the sticking problem is disclosed in commonly assigned U.S. Pat. No. 5,331,454, issued on Jul. 19, 1994 (Hornbeck, xe2x80x9cLow Reset Voltage Process for DMDxe2x80x9d). This patent describes a technique for passivating or lubricating the portion of the landing electrode engaged by the deformed reflector, and/or the portion of the deformed reflector which engages the landing electrode. Passivation is effected by lowering the surface energy of the landing electrode and/or the reflector, which is, in turn, effected by chemically vapor-depositing on the engageable surfaces a monolayer of a long-chain aliphatic halogenated polar compound, such as perfluoroalkyl acid. Objects do not easily, if at all, stick or adhere to low energy surfaces, which are also usually expected to be resistant to sorption thereonto of high surface-energy imparting substances such as water vapor.
Refinements of the passivation method are disclosed in U.S. Pat. No. 5,939,785, issued on Aug. 17, 1999 (Klonis et al., xe2x80x9cMicromechanical Device including Time-release Passivantxe2x80x9d), and U.S. Pat. No. 5,936,758, issued on Aug. 10, 1999 (Fisher et al., xe2x80x9cMethod of Passivating a Micromechanical Device within a Hermetic Packagexe2x80x9d). The method an enclosed source time-releasing a passivant, preferably a molecular sieve or binder impregnated with the passivant. Further, the method is placing a predetermined quantity of the passivant in the package just after device activation, and is then immediately welding a hermetic lid (free of passivant during the welding process) to the package.
Today""s overall package structure for micromechanical devices and method of fabrication is expensive. This fact is in conflict with the market requirements for many applications of micromechanical devices, which put a premium at low device cost and, therefore, low package cost. The state of the art has been advanced by a recent U.S. patent application Ser. No. 09/760,517 entitled xe2x80x9cMolded Package for Micromechanical Devices and Method of Fabricationxe2x80x9d), to which the present invention is related. In that patent application, transfer molding technology is used to produce a low cost package for micromechanical devices, while the surfaces of the micromechanical components are covered by a sheet-like protector during the molding process. This protective cover may be made of water-soluble polyimide. Its application and removal exhibit manufacturing difficulties.
A challenge has now arisen to conceive device structures, fabrication method and equipment features for a low-cost encapsulation technology of micromechanical devices, which provides the molding in thin, chip-scale packages without the need of additional protective cover of the component surfaces. Preferably, this concept should be based on fundamental design solutions flexible enough to be applied for different micromechanical product families and a wide spectrum of design, material and process variations. Manufacturing should be low cost and the devices stable and reliable. Preferably, these innovations should be accomplished while shortening production cycle time and increasing throughput.
According to the present invention, a plastic land-grid array package, a ball-grid array package, and a plastic leaded package for micromechanical components are fabricated by a molding process characterized by lining the cavity surfaces of the top and bottom mold halves with a protective plastic film, which also protects the surfaces of the components during the molding phase, selectively encapsulating the bonding pads and coupling members of the chip while leaving empty space above the components, and attaching a lid over the components. A molding method as well as a molding apparatus are provided compatible with the sensitivity of the micromechanical devices, yet flexible with regard to the technique used to assemble the chip and the substrate. Furthermore, the method disclosed is flexible with regard to the material and the properties of the substrate.
It is an aspect of the present invention to be applicable to a variety of different semiconductor micromechanical devices, for instance actuators, motors, sensors, spatial light modulators, and deformable mirror devices. In all applications, the invention achieves technical advantages as well as significant cost reduction and yield increase.
In a key embodiment of the invention, the micromechanical components are micromirrors for a digital mirror device. In this case, the lid is a plate made of glass or any other material transparent to light. It is another aspect of the present invention to modify the transfer molding technique so that it becomes applicable to encapsulating micromechanical devices and concurrently protects the micromechanical components during the total process (for example against dust and released chemicals), resulting in significantly higher manufacturing yield and enhanced device quality and reliability. Specially developed encapsulation materials of low viscosity and high adhesion can be used.
Another aspect of the invention is to completely embed all sensitive interconnection means (such as bonding wires, ribbons, or solder) in the molded material.
Another aspect of the invention is be applicable to a wide variety of substrates, made either of stiff or of flexible material, such as polymer-based boards or plastic films having multi-level metallization.
Another aspect of the invention is to provide the technology needed for mass-fabricating micromechanical devices in chip-scale or even chip-size packagesxe2x80x94a necessity for many product applications in miniaturized configurations.
Another aspect of the invention is to provide ultra-thin devices free of any voids in the molded portions, and having unusually flat surfaces and high luster. The invention solves these needs by modifying the xe2x80x9c3-Pxe2x80x9d molding technique with its film-protected mold surfaces.
As side benefits of using the xe2x80x9c3-Pxe2x80x9d technology, the pre-assembled devices are protected against contamination and chip cracking, and the mold system avoids down-time and cost due to customary routine cleaning requirements.
Another aspect of the invention is to be applicable to batch processing, including mold chases with multiple cavities, and to easy methods for singulating, trimming and forming the devices.
Another aspect of the invention is to provide flexibility of the package structure relative to the geometry (contour and area) of the semiconductor chip containing the plurality of micromechanical components.
Another aspect of the invention is to employ the precision machining capabilities of today""s mold chase manufacturers in order to obtain the flatness of the molded parts, enabling a lid (for example, glass plate) attachment in a plane parallel to the plane of the plurality of micromechanical components.
These aspects have been achieved by the teachings the invention concerning structure, methods and apparatus suitable for mass production. Various modifications have been employed for the assembly of the semiconductor chips, the design and material of the substrates, and the method of fabrication.
In the first embodiment of the invention, the coupling members connecting the chip bonding pads to the substrate routing line are made of bonding wires or ribbons.
In the second embodiment of the invention, the coupling members connecting the chip bonding pads to the substrate routing lines are made of solder, supplied generally in shapes of balls or bumps.
In the third embodiment of the invention, the substrate is a flexible plastic film having multi-level metal interconnections. The substrate further has terminals for attaching solder balls as means for connecting to external parts.
In the fourth embodiment of the invention, the substrate is a metallic leadframe, which has its external leads trimmed and formed at the end of the assembly and packaging process in order to comply with customer specifications.
The technical advances represented by the invention, as well as the aspects thereof, will become apparent from the following description of the preferred embodiments of the invention, when considered in conjunction with the accompanying drawings and the novel features set forth in the appended claims.